1. Field of the Invention
The present invention relates to CDMA radio channels, and particularly to enhancing performance through dynamically reselecting spreading codes according to current conditions.
2. Description of the Related Art
Typically in wireless telephony, a plurality of base stations (BS) are deployed over the landscape, each of which communicates by radio with a plurality of mobile stations (MS) within its range. Code-division multiple access (CDMA) is a widely used radio transmission medium for wireless telephony. Each user""s signal is spread by multiplying it by a combination of a long code common to all mobile stations communicating with a particular base station and a pseudo-noise (PN) code unique to each of those mobile stations. The users are thus co-occupying all of the available bandwidth. At the mobile station, each user""s signal is recovered by correlating against a combination of the long code and the user""s PN code. This operation is also called despreading. The spreading codes are chosen from a set of orthogonal codes, so that a signal intended for a mobile station will exhibit low cross-correlation with signals intended for other mobile stations. When correlating a particular mobile station""s signal, all other mobile stations"" signals are cancelled due to low cross-correlation.
The transmission delays of each multipath at each mobile station are determined by a searcher, and the signals are delayed accordingly for correlation. The searcher can determine delays for each multipath component, and a known rake receiver (see, for example, CDMA IS-95 for Cellular and PCS, Harte et al, McGraw Hill, New York, N.Y., 1999, ISBN No. 0-07-027070-8, pp. 64-66) can recover signal from several different multipath components by having a correlator for each multipath component, each operating on signal delayed by the appropriate amount. The outputs of the several correlators are then summed. The PN codes are chosen for orthogonality (i.e., low cross-correlation) in order to minimize the apparent noise presented by other mobile stations"" signals when correlating for a particular mobile station.
The third-generation (3G) standard for CDMA calls for wideband CDMA (WCDMA), and includes time-division duplex (TDD). TDD-WCDMA simulations show that the limited auto-correlation and cross-correlation properties of relatively short orthogonal variable spreading factor (OVSF) codes have a significant effect on the channel performance. As transmission conditions change (mobile stations move to different positions, new mobile stations come on, or propagation conditions change) the combination of codes currently in use may become less effective. Although the OVSF codes are chosen for orthogonality, when signals spread by the various OVSF codes are delayed by significantly different amounts, on the order of multiple chip times, the effect of orthogonality diminishes. That is, while correlating for a particular code, another mobile station""s signal exhibiting a significantly different delay may partially correlate, thus raising the noise floor above which the current signal must be detected. There is thus a need to determine optimum code combinations under current conditions when a new mobile station is to be initiated, or when performance for an existing mobile station falls below a predetermined threshold.
To overcome limitations in the prior art described above, the present invention dynamically reselects channel spreading codes according to current transmission delay and signal strength for each multipath component of each channel upon determination that a channel has degraded to an unacceptable error rate, or when a new connection between a base stations and a mobile station is initiated, or when a mobile station is handed off from one base station""s coverage area to another.